/*
 * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
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 *
 */

package java.util.prefs;

import java.util.*;
import java.io.*;
import java.security.AccessController;
import java.security.PrivilegedAction;
// These imports needed only as a workaround for a JavaDoc bug
import java.lang.Integer;
import java.lang.Long;
import java.lang.Float;
import java.lang.Double;

/**
 * This class provides a skeletal implementation of the {@link Preferences}
 * class, greatly easing the task of implementing it.
 *
 * <p><strong>This class is for <tt>Preferences</tt> implementers only.
 * Normal users of the <tt>Preferences</tt> facility should have no need to
 * consult this documentation.  The {@link Preferences} documentation
 * should suffice.</strong>
 *
 * <p>Implementors must override the nine abstract service-provider interface
 * (SPI) methods: {@link #getSpi(String)}, {@link #putSpi(String, String)},
 * {@link #removeSpi(String)}, {@link #childSpi(String)}, {@link
 * #removeNodeSpi()}, {@link #keysSpi()}, {@link #childrenNamesSpi()}, {@link
 * #syncSpi()} and {@link #flushSpi()}.  All of the concrete methods specify
 * precisely how they are implemented atop these SPI methods.  The implementor
 * may, at his discretion, override one or more of the concrete methods if the
 * default implementation is unsatisfactory for any reason, such as
 * performance.
 *
 * <p>The SPI methods fall into three groups concerning exception
 * behavior. The <tt>getSpi</tt> method should never throw exceptions, but it
 * doesn't really matter, as any exception thrown by this method will be
 * intercepted by {@link #get(String, String)}, which will return the specified
 * default value to the caller.  The <tt>removeNodeSpi, keysSpi,
 * childrenNamesSpi, syncSpi</tt> and <tt>flushSpi</tt> methods are specified
 * to throw {@link BackingStoreException}, and the implementation is required
 * to throw this checked exception if it is unable to perform the operation.
 * The exception propagates outward, causing the corresponding API method
 * to fail.
 *
 * <p>The remaining SPI methods {@link #putSpi(String, String)}, {@link
 * #removeSpi(String)} and {@link #childSpi(String)} have more complicated
 * exception behavior.  They are not specified to throw
 * <tt>BackingStoreException</tt>, as they can generally obey their contracts
 * even if the backing store is unavailable.  This is true because they return
 * no information and their effects are not required to become permanent until
 * a subsequent call to {@link Preferences#flush()} or
 * {@link Preferences#sync()}. Generally speaking, these SPI methods should not
 * throw exceptions.  In some implementations, there may be circumstances
 * under which these calls cannot even enqueue the requested operation for
 * later processing.  Even under these circumstances it is generally better to
 * simply ignore the invocation and return, rather than throwing an
 * exception.  Under these circumstances, however, all subsequent invocations
 * of <tt>flush()</tt> and <tt>sync</tt> should return <tt>false</tt>, as
 * returning <tt>true</tt> would imply that all previous operations had
 * successfully been made permanent.
 *
 * <p>There is one circumstance under which <tt>putSpi, removeSpi and
 * childSpi</tt> <i>should</i> throw an exception: if the caller lacks
 * sufficient privileges on the underlying operating system to perform the
 * requested operation.  This will, for instance, occur on most systems
 * if a non-privileged user attempts to modify system preferences.
 * (The required privileges will vary from implementation to
 * implementation.  On some implementations, they are the right to modify the
 * contents of some directory in the file system; on others they are the right
 * to modify contents of some key in a registry.)  Under any of these
 * circumstances, it would generally be undesirable to let the program
 * continue executing as if these operations would become permanent at a later
 * time.  While implementations are not required to throw an exception under
 * these circumstances, they are encouraged to do so.  A {@link
 * SecurityException} would be appropriate.
 *
 * <p>Most of the SPI methods require the implementation to read or write
 * information at a preferences node.  The implementor should beware of the
 * fact that another VM may have concurrently deleted this node from the
 * backing store.  It is the implementation's responsibility to recreate the
 * node if it has been deleted.
 *
 * <p>Implementation note: In Sun's default <tt>Preferences</tt>
 * implementations, the user's identity is inherited from the underlying
 * operating system and does not change for the lifetime of the virtual
 * machine.  It is recognized that server-side <tt>Preferences</tt>
 * implementations may have the user identity change from request to request,
 * implicitly passed to <tt>Preferences</tt> methods via the use of a
 * static {@link ThreadLocal} instance.  Authors of such implementations are
 * <i>strongly</i> encouraged to determine the user at the time preferences
 * are accessed (for example by the {@link #get(String, String)} or {@link
 * #put(String, String)} method) rather than permanently associating a user
 * with each <tt>Preferences</tt> instance.  The latter behavior conflicts
 * with normal <tt>Preferences</tt> usage and would lead to great confusion.
 *
 * @author Josh Bloch
 * @see Preferences
 * @since 1.4
 */
public abstract class AbstractPreferences extends Preferences {

  /**
   * Our name relative to parent.
   */
  private final String name;

  /**
   * Our absolute path name.
   */
  private final String absolutePath;

  /**
   * Our parent node.
   */
  final AbstractPreferences parent;

  /**
   * Our root node.
   */
  private final AbstractPreferences root; // Relative to this node

  /**
   * This field should be <tt>true</tt> if this node did not exist in the
   * backing store prior to the creation of this object.  The field
   * is initialized to false, but may be set to true by a subclass
   * constructor (and should not be modified thereafter).  This field
   * indicates whether a node change event should be fired when
   * creation is complete.
   */
  protected boolean newNode = false;

  /**
   * All known unremoved children of this node.  (This "cache" is consulted
   * prior to calling childSpi() or getChild().
   */
  private Map<String, AbstractPreferences> kidCache = new HashMap<>();

  /**
   * This field is used to keep track of whether or not this node has
   * been removed.  Once it's set to true, it will never be reset to false.
   */
  private boolean removed = false;

  /**
   * Registered preference change listeners.
   */
  private PreferenceChangeListener[] prefListeners =
      new PreferenceChangeListener[0];

  /**
   * Registered node change listeners.
   */
  private NodeChangeListener[] nodeListeners = new NodeChangeListener[0];

  /**
   * An object whose monitor is used to lock this node.  This object
   * is used in preference to the node itself to reduce the likelihood of
   * intentional or unintentional denial of service due to a locked node.
   * To avoid deadlock, a node is <i>never</i> locked by a thread that
   * holds a lock on a descendant of that node.
   */
  protected final Object lock = new Object();

  /**
   * Creates a preference node with the specified parent and the specified
   * name relative to its parent.
   *
   * @param parent the parent of this preference node, or null if this is the root.
   * @param name the name of this preference node, relative to its parent, or <tt>""</tt> if this is
   * the root.
   * @throws IllegalArgumentException if <tt>name</tt> contains a slash (<tt>'/'</tt>),  or
   * <tt>parent</tt> is <tt>null</tt> and name isn't <tt>""</tt>.
   */
  protected AbstractPreferences(AbstractPreferences parent, String name) {
    if (parent == null) {
      if (!name.equals("")) {
        throw new IllegalArgumentException("Root name '" + name +
            "' must be \"\"");
      }
      this.absolutePath = "/";
      root = this;
    } else {
      if (name.indexOf('/') != -1) {
        throw new IllegalArgumentException("Name '" + name +
            "' contains '/'");
      }
      if (name.equals("")) {
        throw new IllegalArgumentException("Illegal name: empty string");
      }

      root = parent.root;
      absolutePath = (parent == root ? "/" + name
          : parent.absolutePath() + "/" + name);
    }
    this.name = name;
    this.parent = parent;
  }

  /**
   * Implements the <tt>put</tt> method as per the specification in
   * {@link Preferences#put(String, String)}.
   *
   * <p>This implementation checks that the key and value are legal,
   * obtains this preference node's lock, checks that the node
   * has not been removed, invokes {@link #putSpi(String, String)}, and if
   * there are any preference change listeners, enqueues a notification
   * event for processing by the event dispatch thread.
   *
   * @param key key with which the specified value is to be associated.
   * @param value value to be associated with the specified key.
   * @throws NullPointerException if key or value is <tt>null</tt>.
   * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds <tt>MAX_KEY_LENGTH</tt> or if
   * <tt>value.length</tt> exceeds <tt>MAX_VALUE_LENGTH</tt>.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  public void put(String key, String value) {
    if (key == null || value == null) {
      throw new NullPointerException();
    }
    if (key.length() > MAX_KEY_LENGTH) {
      throw new IllegalArgumentException("Key too long: " + key);
    }
    if (value.length() > MAX_VALUE_LENGTH) {
      throw new IllegalArgumentException("Value too long: " + value);
    }

    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }

      putSpi(key, value);
      enqueuePreferenceChangeEvent(key, value);
    }
  }

  /**
   * Implements the <tt>get</tt> method as per the specification in
   * {@link Preferences#get(String, String)}.
   *
   * <p>This implementation first checks to see if <tt>key</tt> is
   * <tt>null</tt> throwing a <tt>NullPointerException</tt> if this is
   * the case.  Then it obtains this preference node's lock,
   * checks that the node has not been removed, invokes {@link
   * #getSpi(String)}, and returns the result, unless the <tt>getSpi</tt>
   * invocation returns <tt>null</tt> or throws an exception, in which case
   * this invocation returns <tt>def</tt>.
   *
   * @param key key whose associated value is to be returned.
   * @param def the value to be returned in the event that this preference node has no value
   * associated with <tt>key</tt>.
   * @return the value associated with <tt>key</tt>, or <tt>def</tt> if no value is associated with
   * <tt>key</tt>.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   * @throws NullPointerException if key is <tt>null</tt>.  (A <tt>null</tt> default <i>is</i>
   * permitted.)
   */
  public String get(String key, String def) {
    if (key == null) {
      throw new NullPointerException("Null key");
    }
    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }

      String result = null;
      try {
        result = getSpi(key);
      } catch (Exception e) {
        // Ignoring exception causes default to be returned
      }
      return (result == null ? def : result);
    }
  }

  /**
   * Implements the <tt>remove(String)</tt> method as per the specification
   * in {@link Preferences#remove(String)}.
   *
   * <p>This implementation obtains this preference node's lock,
   * checks that the node has not been removed, invokes
   * {@link #removeSpi(String)} and if there are any preference
   * change listeners, enqueues a notification event for processing by the
   * event dispatch thread.
   *
   * @param key key whose mapping is to be removed from the preference node.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   * @throws NullPointerException {@inheritDoc}.
   */
  public void remove(String key) {
    Objects.requireNonNull(key, "Specified key cannot be null");
    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }

      removeSpi(key);
      enqueuePreferenceChangeEvent(key, null);
    }
  }

  /**
   * Implements the <tt>clear</tt> method as per the specification in
   * {@link Preferences#clear()}.
   *
   * <p>This implementation obtains this preference node's lock,
   * invokes {@link #keys()} to obtain an array of keys, and
   * iterates over the array invoking {@link #remove(String)} on each key.
   *
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  public void clear() throws BackingStoreException {
    synchronized (lock) {
      String[] keys = keys();
      for (int i = 0; i < keys.length; i++) {
        remove(keys[i]);
      }
    }
  }

  /**
   * Implements the <tt>putInt</tt> method as per the specification in
   * {@link Preferences#putInt(String, int)}.
   *
   * <p>This implementation translates <tt>value</tt> to a string with
   * {@link Integer#toString(int)} and invokes {@link #put(String, String)}
   * on the result.
   *
   * @param key key with which the string form of value is to be associated.
   * @param value value whose string form is to be associated with key.
   * @throws NullPointerException if key is <tt>null</tt>.
   * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds <tt>MAX_KEY_LENGTH</tt>.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  public void putInt(String key, int value) {
    put(key, Integer.toString(value));
  }

  /**
   * Implements the <tt>getInt</tt> method as per the specification in
   * {@link Preferences#getInt(String, int)}.
   *
   * <p>This implementation invokes {@link #get(String, String) <tt>get(key,
   * null)</tt>}.  If the return value is non-null, the implementation
   * attempts to translate it to an <tt>int</tt> with
   * {@link Integer#parseInt(String)}.  If the attempt succeeds, the return
   * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
   *
   * @param key key whose associated value is to be returned as an int.
   * @param def the value to be returned in the event that this preference node has no value
   * associated with <tt>key</tt> or the associated value cannot be interpreted as an int.
   * @return the int value represented by the string associated with <tt>key</tt> in this preference
   * node, or <tt>def</tt> if the associated value does not exist or cannot be interpreted as an
   * int.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
   */
  public int getInt(String key, int def) {
    int result = def;
    try {
      String value = get(key, null);
      if (value != null) {
        result = Integer.parseInt(value);
      }
    } catch (NumberFormatException e) {
      // Ignoring exception causes specified default to be returned
    }

    return result;
  }

  /**
   * Implements the <tt>putLong</tt> method as per the specification in
   * {@link Preferences#putLong(String, long)}.
   *
   * <p>This implementation translates <tt>value</tt> to a string with
   * {@link Long#toString(long)} and invokes {@link #put(String, String)}
   * on the result.
   *
   * @param key key with which the string form of value is to be associated.
   * @param value value whose string form is to be associated with key.
   * @throws NullPointerException if key is <tt>null</tt>.
   * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds <tt>MAX_KEY_LENGTH</tt>.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  public void putLong(String key, long value) {
    put(key, Long.toString(value));
  }

  /**
   * Implements the <tt>getLong</tt> method as per the specification in
   * {@link Preferences#getLong(String, long)}.
   *
   * <p>This implementation invokes {@link #get(String, String) <tt>get(key,
   * null)</tt>}.  If the return value is non-null, the implementation
   * attempts to translate it to a <tt>long</tt> with
   * {@link Long#parseLong(String)}.  If the attempt succeeds, the return
   * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
   *
   * @param key key whose associated value is to be returned as a long.
   * @param def the value to be returned in the event that this preference node has no value
   * associated with <tt>key</tt> or the associated value cannot be interpreted as a long.
   * @return the long value represented by the string associated with <tt>key</tt> in this
   * preference node, or <tt>def</tt> if the associated value does not exist or cannot be
   * interpreted as a long.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
   */
  public long getLong(String key, long def) {
    long result = def;
    try {
      String value = get(key, null);
      if (value != null) {
        result = Long.parseLong(value);
      }
    } catch (NumberFormatException e) {
      // Ignoring exception causes specified default to be returned
    }

    return result;
  }

  /**
   * Implements the <tt>putBoolean</tt> method as per the specification in
   * {@link Preferences#putBoolean(String, boolean)}.
   *
   * <p>This implementation translates <tt>value</tt> to a string with
   * {@link String#valueOf(boolean)} and invokes {@link #put(String, String)}
   * on the result.
   *
   * @param key key with which the string form of value is to be associated.
   * @param value value whose string form is to be associated with key.
   * @throws NullPointerException if key is <tt>null</tt>.
   * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds <tt>MAX_KEY_LENGTH</tt>.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  public void putBoolean(String key, boolean value) {
    put(key, String.valueOf(value));
  }

  /**
   * Implements the <tt>getBoolean</tt> method as per the specification in
   * {@link Preferences#getBoolean(String, boolean)}.
   *
   * <p>This implementation invokes {@link #get(String, String) <tt>get(key,
   * null)</tt>}.  If the return value is non-null, it is compared with
   * <tt>"true"</tt> using {@link String#equalsIgnoreCase(String)}.  If the
   * comparison returns <tt>true</tt>, this invocation returns
   * <tt>true</tt>.  Otherwise, the original return value is compared with
   * <tt>"false"</tt>, again using {@link String#equalsIgnoreCase(String)}.
   * If the comparison returns <tt>true</tt>, this invocation returns
   * <tt>false</tt>.  Otherwise, this invocation returns <tt>def</tt>.
   *
   * @param key key whose associated value is to be returned as a boolean.
   * @param def the value to be returned in the event that this preference node has no value
   * associated with <tt>key</tt> or the associated value cannot be interpreted as a boolean.
   * @return the boolean value represented by the string associated with <tt>key</tt> in this
   * preference node, or <tt>def</tt> if the associated value does not exist or cannot be
   * interpreted as a boolean.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
   */
  public boolean getBoolean(String key, boolean def) {
    boolean result = def;
    String value = get(key, null);
    if (value != null) {
      if (value.equalsIgnoreCase("true")) {
        result = true;
      } else if (value.equalsIgnoreCase("false")) {
        result = false;
      }
    }

    return result;
  }

  /**
   * Implements the <tt>putFloat</tt> method as per the specification in
   * {@link Preferences#putFloat(String, float)}.
   *
   * <p>This implementation translates <tt>value</tt> to a string with
   * {@link Float#toString(float)} and invokes {@link #put(String, String)}
   * on the result.
   *
   * @param key key with which the string form of value is to be associated.
   * @param value value whose string form is to be associated with key.
   * @throws NullPointerException if key is <tt>null</tt>.
   * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds <tt>MAX_KEY_LENGTH</tt>.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  public void putFloat(String key, float value) {
    put(key, Float.toString(value));
  }

  /**
   * Implements the <tt>getFloat</tt> method as per the specification in
   * {@link Preferences#getFloat(String, float)}.
   *
   * <p>This implementation invokes {@link #get(String, String) <tt>get(key,
   * null)</tt>}.  If the return value is non-null, the implementation
   * attempts to translate it to an <tt>float</tt> with
   * {@link Float#parseFloat(String)}.  If the attempt succeeds, the return
   * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
   *
   * @param key key whose associated value is to be returned as a float.
   * @param def the value to be returned in the event that this preference node has no value
   * associated with <tt>key</tt> or the associated value cannot be interpreted as a float.
   * @return the float value represented by the string associated with <tt>key</tt> in this
   * preference node, or <tt>def</tt> if the associated value does not exist or cannot be
   * interpreted as a float.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
   */
  public float getFloat(String key, float def) {
    float result = def;
    try {
      String value = get(key, null);
      if (value != null) {
        result = Float.parseFloat(value);
      }
    } catch (NumberFormatException e) {
      // Ignoring exception causes specified default to be returned
    }

    return result;
  }

  /**
   * Implements the <tt>putDouble</tt> method as per the specification in
   * {@link Preferences#putDouble(String, double)}.
   *
   * <p>This implementation translates <tt>value</tt> to a string with
   * {@link Double#toString(double)} and invokes {@link #put(String, String)}
   * on the result.
   *
   * @param key key with which the string form of value is to be associated.
   * @param value value whose string form is to be associated with key.
   * @throws NullPointerException if key is <tt>null</tt>.
   * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds <tt>MAX_KEY_LENGTH</tt>.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  public void putDouble(String key, double value) {
    put(key, Double.toString(value));
  }

  /**
   * Implements the <tt>getDouble</tt> method as per the specification in
   * {@link Preferences#getDouble(String, double)}.
   *
   * <p>This implementation invokes {@link #get(String, String) <tt>get(key,
   * null)</tt>}.  If the return value is non-null, the implementation
   * attempts to translate it to an <tt>double</tt> with
   * {@link Double#parseDouble(String)}.  If the attempt succeeds, the return
   * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
   *
   * @param key key whose associated value is to be returned as a double.
   * @param def the value to be returned in the event that this preference node has no value
   * associated with <tt>key</tt> or the associated value cannot be interpreted as a double.
   * @return the double value represented by the string associated with <tt>key</tt> in this
   * preference node, or <tt>def</tt> if the associated value does not exist or cannot be
   * interpreted as a double.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
   */
  public double getDouble(String key, double def) {
    double result = def;
    try {
      String value = get(key, null);
      if (value != null) {
        result = Double.parseDouble(value);
      }
    } catch (NumberFormatException e) {
      // Ignoring exception causes specified default to be returned
    }

    return result;
  }

  /**
   * Implements the <tt>putByteArray</tt> method as per the specification in
   * {@link Preferences#putByteArray(String, byte[])}.
   *
   * @param key key with which the string form of value is to be associated.
   * @param value value whose string form is to be associated with key.
   * @throws NullPointerException if key or value is <tt>null</tt>.
   * @throws IllegalArgumentException if key.length() exceeds MAX_KEY_LENGTH or if value.length
   * exceeds MAX_VALUE_LENGTH*3/4.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  public void putByteArray(String key, byte[] value) {
    put(key, Base64.byteArrayToBase64(value));
  }

  /**
   * Implements the <tt>getByteArray</tt> method as per the specification in
   * {@link Preferences#getByteArray(String, byte[])}.
   *
   * @param key key whose associated value is to be returned as a byte array.
   * @param def the value to be returned in the event that this preference node has no value
   * associated with <tt>key</tt> or the associated value cannot be interpreted as a byte array.
   * @return the byte array value represented by the string associated with <tt>key</tt> in this
   * preference node, or <tt>def</tt> if the associated value does not exist or cannot be
   * interpreted as a byte array.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.  (A <tt>null</tt> value for
   * <tt>def</tt> <i>is</i> permitted.)
   */
  public byte[] getByteArray(String key, byte[] def) {
    byte[] result = def;
    String value = get(key, null);
    try {
      if (value != null) {
        result = Base64.base64ToByteArray(value);
      }
    } catch (RuntimeException e) {
      // Ignoring exception causes specified default to be returned
    }

    return result;
  }

  /**
   * Implements the <tt>keys</tt> method as per the specification in
   * {@link Preferences#keys()}.
   *
   * <p>This implementation obtains this preference node's lock, checks that
   * the node has not been removed and invokes {@link #keysSpi()}.
   *
   * @return an array of the keys that have an associated value in this preference node.
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  public String[] keys() throws BackingStoreException {
    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }

      return keysSpi();
    }
  }

  /**
   * Implements the <tt>children</tt> method as per the specification in
   * {@link Preferences#childrenNames()}.
   *
   * <p>This implementation obtains this preference node's lock, checks that
   * the node has not been removed, constructs a <tt>TreeSet</tt> initialized
   * to the names of children already cached (the children in this node's
   * "child-cache"), invokes {@link #childrenNamesSpi()}, and adds all of the
   * returned child-names into the set.  The elements of the tree set are
   * dumped into a <tt>String</tt> array using the <tt>toArray</tt> method,
   * and this array is returned.
   *
   * @return the names of the children of this preference node.
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   * @see #cachedChildren()
   */
  public String[] childrenNames() throws BackingStoreException {
    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }

      Set<String> s = new TreeSet<>(kidCache.keySet());
      for (String kid : childrenNamesSpi()) {
        s.add(kid);
      }
      return s.toArray(EMPTY_STRING_ARRAY);
    }
  }

  private static final String[] EMPTY_STRING_ARRAY = new String[0];

  /**
   * Returns all known unremoved children of this node.
   *
   * @return all known unremoved children of this node.
   */
  protected final AbstractPreferences[] cachedChildren() {
    return kidCache.values().toArray(EMPTY_ABSTRACT_PREFS_ARRAY);
  }

  private static final AbstractPreferences[] EMPTY_ABSTRACT_PREFS_ARRAY
      = new AbstractPreferences[0];

  /**
   * Implements the <tt>parent</tt> method as per the specification in
   * {@link Preferences#parent()}.
   *
   * <p>This implementation obtains this preference node's lock, checks that
   * the node has not been removed and returns the parent value that was
   * passed to this node's constructor.
   *
   * @return the parent of this preference node.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  public Preferences parent() {
    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }

      return parent;
    }
  }

  /**
   * Implements the <tt>node</tt> method as per the specification in
   * {@link Preferences#node(String)}.
   *
   * <p>This implementation obtains this preference node's lock and checks
   * that the node has not been removed.  If <tt>path</tt> is <tt>""</tt>,
   * this node is returned; if <tt>path</tt> is <tt>"/"</tt>, this node's
   * root is returned.  If the first character in <tt>path</tt> is
   * not <tt>'/'</tt>, the implementation breaks <tt>path</tt> into
   * tokens and recursively traverses the path from this node to the
   * named node, "consuming" a name and a slash from <tt>path</tt> at
   * each step of the traversal.  At each step, the current node is locked
   * and the node's child-cache is checked for the named node.  If it is
   * not found, the name is checked to make sure its length does not
   * exceed <tt>MAX_NAME_LENGTH</tt>.  Then the {@link #childSpi(String)}
   * method is invoked, and the result stored in this node's child-cache.
   * If the newly created <tt>Preferences</tt> object's {@link #newNode}
   * field is <tt>true</tt> and there are any node change listeners,
   * a notification event is enqueued for processing by the event dispatch
   * thread.
   *
   * <p>When there are no more tokens, the last value found in the
   * child-cache or returned by <tt>childSpi</tt> is returned by this
   * method.  If during the traversal, two <tt>"/"</tt> tokens occur
   * consecutively, or the final token is <tt>"/"</tt> (rather than a name),
   * an appropriate <tt>IllegalArgumentException</tt> is thrown.
   *
   * <p> If the first character of <tt>path</tt> is <tt>'/'</tt>
   * (indicating an absolute path name) this preference node's
   * lock is dropped prior to breaking <tt>path</tt> into tokens, and
   * this method recursively traverses the path starting from the root
   * (rather than starting from this node).  The traversal is otherwise
   * identical to the one described for relative path names.  Dropping
   * the lock on this node prior to commencing the traversal at the root
   * node is essential to avoid the possibility of deadlock, as per the
   * {@link #lock locking invariant}.
   *
   * @param path the path name of the preference node to return.
   * @return the specified preference node.
   * @throws IllegalArgumentException if the path name is invalid (i.e., it contains multiple
   * consecutive slash characters, or ends with a slash character and is more than one character
   * long).
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  public Preferences node(String path) {
    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }
      if (path.equals("")) {
        return this;
      }
      if (path.equals("/")) {
        return root;
      }
      if (path.charAt(0) != '/') {
        return node(new StringTokenizer(path, "/", true));
      }
    }

    // Absolute path.  Note that we've dropped our lock to avoid deadlock
    return root.node(new StringTokenizer(path.substring(1), "/", true));
  }

  /**
   * tokenizer contains <name> {'/' <name>}*
   */
  private Preferences node(StringTokenizer path) {
    String token = path.nextToken();
    if (token.equals("/"))  // Check for consecutive slashes
    {
      throw new IllegalArgumentException("Consecutive slashes in path");
    }
    synchronized (lock) {
      AbstractPreferences child = kidCache.get(token);
      if (child == null) {
        if (token.length() > MAX_NAME_LENGTH) {
          throw new IllegalArgumentException(
              "Node name " + token + " too long");
        }
        child = childSpi(token);
        if (child.newNode) {
          enqueueNodeAddedEvent(child);
        }
        kidCache.put(token, child);
      }
      if (!path.hasMoreTokens()) {
        return child;
      }
      path.nextToken();  // Consume slash
      if (!path.hasMoreTokens()) {
        throw new IllegalArgumentException("Path ends with slash");
      }
      return child.node(path);
    }
  }

  /**
   * Implements the <tt>nodeExists</tt> method as per the specification in
   * {@link Preferences#nodeExists(String)}.
   *
   * <p>This implementation is very similar to {@link #node(String)},
   * except that {@link #getChild(String)} is used instead of {@link
   * #childSpi(String)}.
   *
   * @param path the path name of the node whose existence is to be checked.
   * @return true if the specified node exists.
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   * @throws IllegalArgumentException if the path name is invalid (i.e., it contains multiple
   * consecutive slash characters, or ends with a slash character and is more than one character
   * long).
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method and <tt>pathname</tt> is not the empty string (<tt>""</tt>).
   */
  public boolean nodeExists(String path)
      throws BackingStoreException {
    synchronized (lock) {
      if (path.equals("")) {
        return !removed;
      }
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }
      if (path.equals("/")) {
        return true;
      }
      if (path.charAt(0) != '/') {
        return nodeExists(new StringTokenizer(path, "/", true));
      }
    }

    // Absolute path.  Note that we've dropped our lock to avoid deadlock
    return root.nodeExists(new StringTokenizer(path.substring(1), "/",
        true));
  }

  /**
   * tokenizer contains <name> {'/' <name>}*
   */
  private boolean nodeExists(StringTokenizer path)
      throws BackingStoreException {
    String token = path.nextToken();
    if (token.equals("/"))  // Check for consecutive slashes
    {
      throw new IllegalArgumentException("Consecutive slashes in path");
    }
    synchronized (lock) {
      AbstractPreferences child = kidCache.get(token);
      if (child == null) {
        child = getChild(token);
      }
      if (child == null) {
        return false;
      }
      if (!path.hasMoreTokens()) {
        return true;
      }
      path.nextToken();  // Consume slash
      if (!path.hasMoreTokens()) {
        throw new IllegalArgumentException("Path ends with slash");
      }
      return child.nodeExists(path);
    }
  }

  /**
   * Implements the <tt>removeNode()</tt> method as per the specification in
   * {@link Preferences#removeNode()}.
   *
   * <p>This implementation checks to see that this node is the root; if so,
   * it throws an appropriate exception.  Then, it locks this node's parent,
   * and calls a recursive helper method that traverses the subtree rooted at
   * this node.  The recursive method locks the node on which it was called,
   * checks that it has not already been removed, and then ensures that all
   * of its children are cached: The {@link #childrenNamesSpi()} method is
   * invoked and each returned child name is checked for containment in the
   * child-cache.  If a child is not already cached, the {@link
   * #childSpi(String)} method is invoked to create a <tt>Preferences</tt>
   * instance for it, and this instance is put into the child-cache.  Then
   * the helper method calls itself recursively on each node contained in its
   * child-cache.  Next, it invokes {@link #removeNodeSpi()}, marks itself
   * as removed, and removes itself from its parent's child-cache.  Finally,
   * if there are any node change listeners, it enqueues a notification
   * event for processing by the event dispatch thread.
   *
   * <p>Note that the helper method is always invoked with all ancestors up
   * to the "closest non-removed ancestor" locked.
   *
   * @throws IllegalStateException if this node (or an ancestor) has already been removed with the
   * {@link #removeNode()} method.
   * @throws UnsupportedOperationException if this method is invoked on the root node.
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   */
  public void removeNode() throws BackingStoreException {
    if (this == root) {
      throw new UnsupportedOperationException("Can't remove the root!");
    }
    synchronized (parent.lock) {
      removeNode2();
      parent.kidCache.remove(name);
    }
  }

  /*
   * Called with locks on all nodes on path from parent of "removal root"
   * to this (including the former but excluding the latter).
   */
  private void removeNode2() throws BackingStoreException {
    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node already removed.");
      }

      // Ensure that all children are cached
      String[] kidNames = childrenNamesSpi();
      for (int i = 0; i < kidNames.length; i++) {
        if (!kidCache.containsKey(kidNames[i])) {
          kidCache.put(kidNames[i], childSpi(kidNames[i]));
        }
      }

      // Recursively remove all cached children
      for (Iterator<AbstractPreferences> i = kidCache.values().iterator();
          i.hasNext(); ) {
        try {
          i.next().removeNode2();
          i.remove();
        } catch (BackingStoreException x) {
        }
      }

      // Now we have no descendants - it's time to die!
      removeNodeSpi();
      removed = true;
      parent.enqueueNodeRemovedEvent(this);
    }
  }

  /**
   * Implements the <tt>name</tt> method as per the specification in
   * {@link Preferences#name()}.
   *
   * <p>This implementation merely returns the name that was
   * passed to this node's constructor.
   *
   * @return this preference node's name, relative to its parent.
   */
  public String name() {
    return name;
  }

  /**
   * Implements the <tt>absolutePath</tt> method as per the specification in
   * {@link Preferences#absolutePath()}.
   *
   * <p>This implementation merely returns the absolute path name that
   * was computed at the time that this node was constructed (based on
   * the name that was passed to this node's constructor, and the names
   * that were passed to this node's ancestors' constructors).
   *
   * @return this preference node's absolute path name.
   */
  public String absolutePath() {
    return absolutePath;
  }

  /**
   * Implements the <tt>isUserNode</tt> method as per the specification in
   * {@link Preferences#isUserNode()}.
   *
   * <p>This implementation compares this node's root node (which is stored
   * in a private field) with the value returned by
   * {@link Preferences#userRoot()}.  If the two object references are
   * identical, this method returns true.
   *
   * @return <tt>true</tt> if this preference node is in the user preference tree, <tt>false</tt> if
   * it's in the system preference tree.
   */
  public boolean isUserNode() {
    return AccessController.doPrivileged(
        new PrivilegedAction<Boolean>() {
          public Boolean run() {
            return root == Preferences.userRoot();
          }
        }).booleanValue();
  }

  public void addPreferenceChangeListener(PreferenceChangeListener pcl) {
    if (pcl == null) {
      throw new NullPointerException("Change listener is null.");
    }
    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }

      // Copy-on-write
      PreferenceChangeListener[] old = prefListeners;
      prefListeners = new PreferenceChangeListener[old.length + 1];
      System.arraycopy(old, 0, prefListeners, 0, old.length);
      prefListeners[old.length] = pcl;
    }
    startEventDispatchThreadIfNecessary();
  }

  public void removePreferenceChangeListener(PreferenceChangeListener pcl) {
    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }
      if ((prefListeners == null) || (prefListeners.length == 0)) {
        throw new IllegalArgumentException("Listener not registered.");
      }

      // Copy-on-write
      PreferenceChangeListener[] newPl =
          new PreferenceChangeListener[prefListeners.length - 1];
      int i = 0;
      while (i < newPl.length && prefListeners[i] != pcl) {
        newPl[i] = prefListeners[i++];
      }

      if (i == newPl.length && prefListeners[i] != pcl) {
        throw new IllegalArgumentException("Listener not registered.");
      }
      while (i < newPl.length) {
        newPl[i] = prefListeners[++i];
      }
      prefListeners = newPl;
    }
  }

  public void addNodeChangeListener(NodeChangeListener ncl) {
    if (ncl == null) {
      throw new NullPointerException("Change listener is null.");
    }
    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }

      // Copy-on-write
      if (nodeListeners == null) {
        nodeListeners = new NodeChangeListener[1];
        nodeListeners[0] = ncl;
      } else {
        NodeChangeListener[] old = nodeListeners;
        nodeListeners = new NodeChangeListener[old.length + 1];
        System.arraycopy(old, 0, nodeListeners, 0, old.length);
        nodeListeners[old.length] = ncl;
      }
    }
    startEventDispatchThreadIfNecessary();
  }

  public void removeNodeChangeListener(NodeChangeListener ncl) {
    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed.");
      }
      if ((nodeListeners == null) || (nodeListeners.length == 0)) {
        throw new IllegalArgumentException("Listener not registered.");
      }

      // Copy-on-write
      int i = 0;
      while (i < nodeListeners.length && nodeListeners[i] != ncl) {
        i++;
      }
      if (i == nodeListeners.length) {
        throw new IllegalArgumentException("Listener not registered.");
      }
      NodeChangeListener[] newNl =
          new NodeChangeListener[nodeListeners.length - 1];
      if (i != 0) {
        System.arraycopy(nodeListeners, 0, newNl, 0, i);
      }
      if (i != newNl.length) {
        System.arraycopy(nodeListeners, i + 1,
            newNl, i, newNl.length - i);
      }
      nodeListeners = newNl;
    }
  }

  // "SPI" METHODS

  /**
   * Put the given key-value association into this preference node.  It is
   * guaranteed that <tt>key</tt> and <tt>value</tt> are non-null and of
   * legal length.  Also, it is guaranteed that this node has not been
   * removed.  (The implementor needn't check for any of these things.)
   *
   * <p>This method is invoked with the lock on this node held.
   *
   * @param key the key
   * @param value the value
   */
  protected abstract void putSpi(String key, String value);

  /**
   * Return the value associated with the specified key at this preference
   * node, or <tt>null</tt> if there is no association for this key, or the
   * association cannot be determined at this time.  It is guaranteed that
   * <tt>key</tt> is non-null.  Also, it is guaranteed that this node has
   * not been removed.  (The implementor needn't check for either of these
   * things.)
   *
   * <p> Generally speaking, this method should not throw an exception
   * under any circumstances.  If, however, if it does throw an exception,
   * the exception will be intercepted and treated as a <tt>null</tt>
   * return value.
   *
   * <p>This method is invoked with the lock on this node held.
   *
   * @param key the key
   * @return the value associated with the specified key at this preference node, or <tt>null</tt>
   * if there is no association for this key, or the association cannot be determined at this time.
   */
  protected abstract String getSpi(String key);

  /**
   * Remove the association (if any) for the specified key at this
   * preference node.  It is guaranteed that <tt>key</tt> is non-null.
   * Also, it is guaranteed that this node has not been removed.
   * (The implementor needn't check for either of these things.)
   *
   * <p>This method is invoked with the lock on this node held.
   *
   * @param key the key
   */
  protected abstract void removeSpi(String key);

  /**
   * Removes this preference node, invalidating it and any preferences that
   * it contains.  The named child will have no descendants at the time this
   * invocation is made (i.e., the {@link Preferences#removeNode()} method
   * invokes this method repeatedly in a bottom-up fashion, removing each of
   * a node's descendants before removing the node itself).
   *
   * <p>This method is invoked with the lock held on this node and its
   * parent (and all ancestors that are being removed as a
   * result of a single invocation to {@link Preferences#removeNode()}).
   *
   * <p>The removal of a node needn't become persistent until the
   * <tt>flush</tt> method is invoked on this node (or an ancestor).
   *
   * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
   * will propagate out beyond the enclosing {@link #removeNode()}
   * invocation.
   *
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   */
  protected abstract void removeNodeSpi() throws BackingStoreException;

  /**
   * Returns all of the keys that have an associated value in this
   * preference node.  (The returned array will be of size zero if
   * this node has no preferences.)  It is guaranteed that this node has not
   * been removed.
   *
   * <p>This method is invoked with the lock on this node held.
   *
   * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
   * will propagate out beyond the enclosing {@link #keys()} invocation.
   *
   * @return an array of the keys that have an associated value in this preference node.
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   */
  protected abstract String[] keysSpi() throws BackingStoreException;

  /**
   * Returns the names of the children of this preference node.  (The
   * returned array will be of size zero if this node has no children.)
   * This method need not return the names of any nodes already cached,
   * but may do so without harm.
   *
   * <p>This method is invoked with the lock on this node held.
   *
   * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
   * will propagate out beyond the enclosing {@link #childrenNames()}
   * invocation.
   *
   * @return an array containing the names of the children of this preference node.
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   */
  protected abstract String[] childrenNamesSpi()
      throws BackingStoreException;

  /**
   * Returns the named child if it exists, or <tt>null</tt> if it does not.
   * It is guaranteed that <tt>nodeName</tt> is non-null, non-empty,
   * does not contain the slash character ('/'), and is no longer than
   * {@link #MAX_NAME_LENGTH} characters.  Also, it is guaranteed
   * that this node has not been removed.  (The implementor needn't check
   * for any of these things if he chooses to override this method.)
   *
   * <p>Finally, it is guaranteed that the named node has not been returned
   * by a previous invocation of this method or {@link #childSpi} after the
   * last time that it was removed.  In other words, a cached value will
   * always be used in preference to invoking this method.  (The implementor
   * needn't maintain his own cache of previously returned children if he
   * chooses to override this method.)
   *
   * <p>This implementation obtains this preference node's lock, invokes
   * {@link #childrenNames()} to get an array of the names of this node's
   * children, and iterates over the array comparing the name of each child
   * with the specified node name.  If a child node has the correct name,
   * the {@link #childSpi(String)} method is invoked and the resulting
   * node is returned.  If the iteration completes without finding the
   * specified name, <tt>null</tt> is returned.
   *
   * @param nodeName name of the child to be searched for.
   * @return the named child if it exists, or null if it does not.
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   */
  protected AbstractPreferences getChild(String nodeName)
      throws BackingStoreException {
    synchronized (lock) {
      // assert kidCache.get(nodeName)==null;
      String[] kidNames = childrenNames();
      for (int i = 0; i < kidNames.length; i++) {
        if (kidNames[i].equals(nodeName)) {
          return childSpi(kidNames[i]);
        }
      }
    }
    return null;
  }

  /**
   * Returns the named child of this preference node, creating it if it does
   * not already exist.  It is guaranteed that <tt>name</tt> is non-null,
   * non-empty, does not contain the slash character ('/'), and is no longer
   * than {@link #MAX_NAME_LENGTH} characters.  Also, it is guaranteed that
   * this node has not been removed.  (The implementor needn't check for any
   * of these things.)
   *
   * <p>Finally, it is guaranteed that the named node has not been returned
   * by a previous invocation of this method or {@link #getChild(String)}
   * after the last time that it was removed.  In other words, a cached
   * value will always be used in preference to invoking this method.
   * Subclasses need not maintain their own cache of previously returned
   * children.
   *
   * <p>The implementer must ensure that the returned node has not been
   * removed.  If a like-named child of this node was previously removed, the
   * implementer must return a newly constructed <tt>AbstractPreferences</tt>
   * node; once removed, an <tt>AbstractPreferences</tt> node
   * cannot be "resuscitated."
   *
   * <p>If this method causes a node to be created, this node is not
   * guaranteed to be persistent until the <tt>flush</tt> method is
   * invoked on this node or one of its ancestors (or descendants).
   *
   * <p>This method is invoked with the lock on this node held.
   *
   * @param name The name of the child node to return, relative to this preference node.
   * @return The named child node.
   */
  protected abstract AbstractPreferences childSpi(String name);

  /**
   * Returns the absolute path name of this preferences node.
   */
  public String toString() {
    return (this.isUserNode() ? "User" : "System") +
        " Preference Node: " + this.absolutePath();
  }

  /**
   * Implements the <tt>sync</tt> method as per the specification in
   * {@link Preferences#sync()}.
   *
   * <p>This implementation calls a recursive helper method that locks this
   * node, invokes syncSpi() on it, unlocks this node, and recursively
   * invokes this method on each "cached child."  A cached child is a child
   * of this node that has been created in this VM and not subsequently
   * removed.  In effect, this method does a depth first traversal of the
   * "cached subtree" rooted at this node, calling syncSpi() on each node in
   * the subTree while only that node is locked. Note that syncSpi() is
   * invoked top-down.
   *
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   * @throws IllegalStateException if this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   * @see #flush()
   */
  public void sync() throws BackingStoreException {
    sync2();
  }

  private void sync2() throws BackingStoreException {
    AbstractPreferences[] cachedKids;

    synchronized (lock) {
      if (removed) {
        throw new IllegalStateException("Node has been removed");
      }
      syncSpi();
      cachedKids = cachedChildren();
    }

    for (int i = 0; i < cachedKids.length; i++) {
      cachedKids[i].sync2();
    }
  }

  /**
   * This method is invoked with this node locked.  The contract of this
   * method is to synchronize any cached preferences stored at this node
   * with any stored in the backing store.  (It is perfectly possible that
   * this node does not exist on the backing store, either because it has
   * been deleted by another VM, or because it has not yet been created.)
   * Note that this method should <i>not</i> synchronize the preferences in
   * any subnodes of this node.  If the backing store naturally syncs an
   * entire subtree at once, the implementer is encouraged to override
   * sync(), rather than merely overriding this method.
   *
   * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
   * will propagate out beyond the enclosing {@link #sync()} invocation.
   *
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   */
  protected abstract void syncSpi() throws BackingStoreException;

  /**
   * Implements the <tt>flush</tt> method as per the specification in
   * {@link Preferences#flush()}.
   *
   * <p>This implementation calls a recursive helper method that locks this
   * node, invokes flushSpi() on it, unlocks this node, and recursively
   * invokes this method on each "cached child."  A cached child is a child
   * of this node that has been created in this VM and not subsequently
   * removed.  In effect, this method does a depth first traversal of the
   * "cached subtree" rooted at this node, calling flushSpi() on each node in
   * the subTree while only that node is locked. Note that flushSpi() is
   * invoked top-down.
   *
   * <p> If this method is invoked on a node that has been removed with
   * the {@link #removeNode()} method, flushSpi() is invoked on this node,
   * but not on others.
   *
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   * @see #flush()
   */
  public void flush() throws BackingStoreException {
    flush2();
  }

  private void flush2() throws BackingStoreException {
    AbstractPreferences[] cachedKids;

    synchronized (lock) {
      flushSpi();
      if (removed) {
        return;
      }
      cachedKids = cachedChildren();
    }

    for (int i = 0; i < cachedKids.length; i++) {
      cachedKids[i].flush2();
    }
  }

  /**
   * This method is invoked with this node locked.  The contract of this
   * method is to force any cached changes in the contents of this
   * preference node to the backing store, guaranteeing their persistence.
   * (It is perfectly possible that this node does not exist on the backing
   * store, either because it has been deleted by another VM, or because it
   * has not yet been created.)  Note that this method should <i>not</i>
   * flush the preferences in any subnodes of this node.  If the backing
   * store naturally flushes an entire subtree at once, the implementer is
   * encouraged to override flush(), rather than merely overriding this
   * method.
   *
   * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
   * will propagate out beyond the enclosing {@link #flush()} invocation.
   *
   * @throws BackingStoreException if this operation cannot be completed due to a failure in the
   * backing store, or inability to communicate with it.
   */
  protected abstract void flushSpi() throws BackingStoreException;

  /**
   * Returns <tt>true</tt> iff this node (or an ancestor) has been
   * removed with the {@link #removeNode()} method.  This method
   * locks this node prior to returning the contents of the private
   * field used to track this state.
   *
   * @return <tt>true</tt> iff this node (or an ancestor) has been removed with the {@link
   * #removeNode()} method.
   */
  protected boolean isRemoved() {
    synchronized (lock) {
      return removed;
    }
  }

  /**
   * Queue of pending notification events.  When a preference or node
   * change event for which there are one or more listeners occurs,
   * it is placed on this queue and the queue is notified.  A background
   * thread waits on this queue and delivers the events.  This decouples
   * event delivery from preference activity, greatly simplifying
   * locking and reducing opportunity for deadlock.
   */
  private static final List<EventObject> eventQueue = new LinkedList<>();

  /**
   * These two classes are used to distinguish NodeChangeEvents on
   * eventQueue so the event dispatch thread knows whether to call
   * childAdded or childRemoved.
   */
  private class NodeAddedEvent extends NodeChangeEvent {

    private static final long serialVersionUID = -6743557530157328528L;

    NodeAddedEvent(Preferences parent, Preferences child) {
      super(parent, child);
    }
  }

  private class NodeRemovedEvent extends NodeChangeEvent {

    private static final long serialVersionUID = 8735497392918824837L;

    NodeRemovedEvent(Preferences parent, Preferences child) {
      super(parent, child);
    }
  }

  /**
   * A single background thread ("the event notification thread") monitors
   * the event queue and delivers events that are placed on the queue.
   */
  private static class EventDispatchThread extends Thread {

    public void run() {
      while (true) {
        // Wait on eventQueue till an event is present
        EventObject event = null;
        synchronized (eventQueue) {
          try {
            while (eventQueue.isEmpty()) {
              eventQueue.wait();
            }
            event = eventQueue.remove(0);
          } catch (InterruptedException e) {
            // XXX Log "Event dispatch thread interrupted. Exiting"
            return;
          }
        }

        // Now we have event & hold no locks; deliver evt to listeners
        AbstractPreferences src = (AbstractPreferences) event.getSource();
        if (event instanceof PreferenceChangeEvent) {
          PreferenceChangeEvent pce = (PreferenceChangeEvent) event;
          PreferenceChangeListener[] listeners = src.prefListeners();
          for (int i = 0; i < listeners.length; i++) {
            listeners[i].preferenceChange(pce);
          }
        } else {
          NodeChangeEvent nce = (NodeChangeEvent) event;
          NodeChangeListener[] listeners = src.nodeListeners();
          if (nce instanceof NodeAddedEvent) {
            for (int i = 0; i < listeners.length; i++) {
              listeners[i].childAdded(nce);
            }
          } else {
            // assert nce instanceof NodeRemovedEvent;
            for (int i = 0; i < listeners.length; i++) {
              listeners[i].childRemoved(nce);
            }
          }
        }
      }
    }
  }

  private static Thread eventDispatchThread = null;

  /**
   * This method starts the event dispatch thread the first time it
   * is called.  The event dispatch thread will be started only
   * if someone registers a listener.
   */
  private static synchronized void startEventDispatchThreadIfNecessary() {
    if (eventDispatchThread == null) {
      // XXX Log "Starting event dispatch thread"
      eventDispatchThread = new EventDispatchThread();
      eventDispatchThread.setDaemon(true);
      eventDispatchThread.start();
    }
  }

  /**
   * Return this node's preference/node change listeners.  Even though
   * we're using a copy-on-write lists, we use synchronized accessors to
   * ensure information transmission from the writing thread to the
   * reading thread.
   */
  PreferenceChangeListener[] prefListeners() {
    synchronized (lock) {
      return prefListeners;
    }
  }

  NodeChangeListener[] nodeListeners() {
    synchronized (lock) {
      return nodeListeners;
    }
  }

  /**
   * Enqueue a preference change event for delivery to registered
   * preference change listeners unless there are no registered
   * listeners.  Invoked with this.lock held.
   */
  private void enqueuePreferenceChangeEvent(String key, String newValue) {
    if (prefListeners.length != 0) {
      synchronized (eventQueue) {
        eventQueue.add(new PreferenceChangeEvent(this, key, newValue));
        eventQueue.notify();
      }
    }
  }

  /**
   * Enqueue a "node added" event for delivery to registered node change
   * listeners unless there are no registered listeners.  Invoked with
   * this.lock held.
   */
  private void enqueueNodeAddedEvent(Preferences child) {
    if (nodeListeners.length != 0) {
      synchronized (eventQueue) {
        eventQueue.add(new NodeAddedEvent(this, child));
        eventQueue.notify();
      }
    }
  }

  /**
   * Enqueue a "node removed" event for delivery to registered node change
   * listeners unless there are no registered listeners.  Invoked with
   * this.lock held.
   */
  private void enqueueNodeRemovedEvent(Preferences child) {
    if (nodeListeners.length != 0) {
      synchronized (eventQueue) {
        eventQueue.add(new NodeRemovedEvent(this, child));
        eventQueue.notify();
      }
    }
  }

  /**
   * Implements the <tt>exportNode</tt> method as per the specification in
   * {@link Preferences#exportNode(OutputStream)}.
   *
   * @param os the output stream on which to emit the XML document.
   * @throws IOException if writing to the specified output stream results in an
   * <tt>IOException</tt>.
   * @throws BackingStoreException if preference data cannot be read from backing store.
   */
  public void exportNode(OutputStream os)
      throws IOException, BackingStoreException {
    XmlSupport.export(os, this, false);
  }

  /**
   * Implements the <tt>exportSubtree</tt> method as per the specification in
   * {@link Preferences#exportSubtree(OutputStream)}.
   *
   * @param os the output stream on which to emit the XML document.
   * @throws IOException if writing to the specified output stream results in an
   * <tt>IOException</tt>.
   * @throws BackingStoreException if preference data cannot be read from backing store.
   */
  public void exportSubtree(OutputStream os)
      throws IOException, BackingStoreException {
    XmlSupport.export(os, this, true);
  }
}
